"Reply to this email" might not be a bad idea for some basic "making an explicit action" check.
Arguably if the question is mail delivery, email already has a complex system of delivery failure reporting. Just trust that, like email programs have been doing since email was invented? "No failure is a success," doesn't have an explicit acknowledgement action from a user, but it is still a meaningful criteria.
"Reply to this email" might not be a bad idea for some basic "making an explicit action" check.
Arguably if the question is mail delivery, email already has a complex system of delivery failure reporting. Just trust that, like email programs have been doing since email was invented? "No failure is a success," doesn't have an explicit acknowledgement action from a user, but it is still a meaningful criteria.